The present invention relates to a proton conductor and a method for manufacturing the proton conductor, and also relates to an electrochemical device.
On-going efforts continue in an attempt to develop the fuel cell because attention is focused on the fuel cell as an environment-oriented electric energy generating system for the next generation from the reasons such as its high efficiency and cleanliness with respect to power generation.
Fuel cell can be roughly divided according to the types of proton conductor used in it because the operating temperature and the condition in use exert strong influence on the property of the proton conductor. Because the property of the proton conductor in use gives strong influence on performance characteristics of the fuel cell, it is essential to improve performance of the proton conductor in order to have the fuel cell with better performance characteristics.
In general, in the temperature range from normal temperature to lower than 100° C., proton-conductive macromolecular film is used, which has solid macromolecular film. Typical examples include NAFION (trade name; DU PONT De NEMOURS & CO.) or GORE FILM (GORE & ASSOCIATES). These are perfluorosulfonic acid resins, and efforts continue in an attempt to make modifications and improvements of these types of products. In addition to these perfluoro type resins, hydrocarbon type macromolecular film have been known in recent years.
Also, as relatively new types of inorganic metal oxide type proton conductor, polymolybdic acids or oxides having a large amount of conductor, polymolybdic acids or oxides having a large amount of water of hydration such as H3M12PO40.29H2O (M=Mo,W) or Sb2O5.nH2O are known.
When these macromolecular materials or hydrated compounds are placed under humid conditions, they exhibit high proton conductive property near normal temperature. Specifically, if an example is taken on perfluoro-sulfonic acid resin, protons electrolytically dissociated from sulfonic group are bonded with moisture which is extensively contained in macromolecular matrix (hydrogen bonding), and protonized water, i.e. oxonium ions (H3O+) are generated. Because protons in form of oxonium ions can smoothly migrate in macromolecular matrix, matrix material of this type can provide considerably high proton conductive effect even at normal temperature.
On the other hand, a proton conductor with conductive mechanism entirely different from these has been recently developed. Specifically, it has been found that complex metal oxides with perovskite structure such as SrCeO3 doped with Yb have proton conductivity even when moisture is not used as migration medium. It appears that, in the complex metal oxides, protons are conducted through channel between oxygen ions, which form skeletons of the perovskite structure.
In this case, it is not that conductive protons are present in the complex metal oxides from the beginning. It is believed that, when the perovskite structure is brought into contact with vapor contained in atmospheric gas in the surroundings, water molecules at high temperature react with oxygen-lacking portion in the perovskite structure by doping, and protons are generated only when this reaction takes place.
However, in the proton conductor as described above, there are a number of problems.
For example, the matrix material such as perfluorosulfonic acid resin must be continuously maintained under sufficiently humid condition during use in order to keep proton conductive property at high level. Also, for the purpose of preventing freezing or boiling of water contained in the matrix, the range of the operating temperature should not be wide.
In case of inorganic metal oxide proton conductor such as H3M12PO40.29H2O (M=Mo,W) or Sb2O5.nH2O, temperature must be maintained high in order to keep structural water contained therein so that significant proton conduction is carried out. Also, in a certain type of perovskite oxide such as SrCeO3, operating temperature must be kept at high level, i.e. 500° C. or higher. When humidity is low, proton conductivity rapidly decreases.
As described above, the conventional type proton conductor depends much on the atmospheric conditions, e.g. moisture must be supplied or vapor is needed. Moreover, there are problems that operating temperature is too high or temperature range is too narrow.
Therefore, humidifier or other types of accessory equipment or device are needed for a system such as fuel cell. This unavoidably requires the designing of the system in larger size or higher cost for system construction. A need, therefore, exists to provide an improved proton conductor and methods of making and using same.